Inkjet printing is a recording method in which liquid droplets of an inkjet ink that have been discharged from an inkjet head are jetted directly onto a recording medium to form text or images. Unlike conventional plate printing techniques such as offset printing and gravure printing, inkjet printing does not require platemaking. Accordingly, inkjet printing has the characteristics of being readily adaptable to variable printing, providing simple operation and adjustment of the printing apparatus, and producing little noise during printing, and as a result, inkjet printing is no longer limited to uses in offices and households, but is also showing increased demand in industrial applications in recent years.
The inkjet inks used in inkjet printing are classified into solvent inks, aqueous inks and UV-curable inks and the like depending on the ink composition. In recent years, there has been a growing movement to restrict the use of organic solvents and photosensitive monomers that can be harmful to people and the environment. As a result, there is a growing demand for the replacement of solvent inks and UV-curable inks that use these types of restricted materials with aqueous inks.
A representative aqueous inkjet ink (hereafter also referred to as simply an “ink”) contains water as the main component, and in order to control the wetting of the recording medium and the drying properties, typically also contains an added water-soluble organic solvent such as glycerol or a glycol. When an aqueous inkjet ink composed of these liquid components is used for printing (applying the ink to form) a pattern of text and/or images onto a recording medium, the ink dries as a result of the liquid components penetrating into the recording medium and/or evaporating from the recording medium, thereby fixing the ink to the recording medium and forming the pattern.
On the other hand, a large variety of recording media exist for use with inkjet printing, from media having high permeability such as high-quality papers and recycled papers, through to media that exhibit little or no absorption such as coated papers, art papers and plastic films. Generally, in the case of printing to recording media having high absorption properties such as high-quality papers and recycled papers, aqueous inkjet inks can be used for printing (image formation of) images having a practically usable image quality. In contrast, when printing to recording media that exhibit little or no absorption such as coated papers, art papers and plastic films, bleeding between colors tends to occur very readily. Accordingly, when printing to recording media that exhibit little or no absorption, the image quality tends to deteriorate, and obtaining image quality of a practically usable level is difficult.
Due to these issues, in order to enable further expansion in the demand for inkjet printing, the development of an aqueous inkjet ink that can be used favorably on all manner of recording media is a large challenge for those in this field.
One known technique for improving image quality when performing inkjet printing using aqueous inkjet inks is a method that involves treating the recording medium with a pretreatment solution.
There are generally two known types of pretreatment solutions for use with aqueous inkjet inks, those that form an ink-receiving layer and those that form an ink aggregation layer. In other words, there are pretreatment solutions that form a layer (ink-receiving layer) that absorbs the liquid components in the aforementioned aqueous inkjet ink and improves the drying properties (see Patent Documents 1 to 4), and pretreatment solutions that form a layer (ink aggregation layer) that causes intentional aggregation of the solid components such as colorants and resins contained in the aqueous inkjet ink, thereby preventing bleeding between liquid droplets and color irregularities and improving the image quality (see Patent Documents 5 and 6). However, there are still no pretreatment solutions for aqueous inkjet inks that are entirely satisfactory, and further improvements would be desirable.
For example, in the case of an ink-receiving layer, if the layer receives a large amount of ink at once, then swelling of the ink-receiving layer can cause waviness or curling (rolling) of the recording medium, or cracking of the image. Further, bleeding or color irregularities caused by exceeding the amount of ink able to be received by the ink-receiving layer, and a deterioration in print density as a result of absorption of ink components by the receiving layer can also occur.
In contrast, in the case of an ink aggregation layer, because basically no absorption of the liquid components occurs, these phenomena are unlikely to occur. However, because the ink aggregation layer exhibits poor absorption of liquid components, if a large amount of ink is applied at once, then considerable time is required for the liquid components to dry, meaning waviness of the printed matter and bleeding become more likely. Further, if the ink aggregation layer does not have a uniform thickness, then there is a possibility of a deterioration in image quality, and the occurrence of other problems associated with localized deterioration in the drying properties.
In other words, in those regions where the thickness of the ink aggregation layer is small, the ink aggregation effect is reduced, and bleeding and color irregularities tend to occur. In contrast, in those regions where the thickness is large, although bleeding and color irregularities can be suppressed, there is a possibility that the drying properties of the pretreatment solution and the ink applied thereon may deteriorate. When the drying properties are poor, problems such as adhesion of the pretreatment solution and the ink to the inside of the coating apparatus, and set-off that occurs when the recording media are stacked following printing become more likely. Image deterioration and other problems caused by a deterioration in drying properties are particularly likely to occur when printing at high speed and at high recording resolution, and can sometimes lead to a large deterioration in the quality of the printed matter. Accordingly, when employing a pretreatment solution for forming an ink aggregation layer, excellent drying properties and the ability to form a uniform coating are very important.